1. Field of the Invention
This invention relates to an electroluminescent device (hereinafter referred to as "EL-device") and, more particularly, to an EL-device comprising a layer having an electroluminescent function (hereinafter referred to as "EL-function") constituted of a combination of two types of thin films composed of organic compounds having electrochemical characteristics different from each other.
2. Description of the Prior Art
An EL-device is constituted of a luminescent layer composed of a material having EL-function, that is, a material capable of emitting light when placed in an electric field, sandwiched between two electrodes, and is a luminescent device which can convert electric energy directly into light when a voltage is applied across the electrodes. The EL-device is different from conventional luminescent systems such as an incandescent lamp which emits light by making a filament incandescent, a fluorescent lamp where a gas excited electrically imparts an energy to a fluorescent substance to emit light, and the like. Therefore, EL-devices can be expected to be used as constitution materials for lamps and display mediums used for displaying lines, figures, images and the like of various shapes such as thin panel, belt, cylinder and the like, or further, a luminescent body of a large area such as panel lamps and the like. These points of EL-device draw attention.
Depending on the luminescent mechanism, EL-devices are generally classified into two classes, i.e. an intrinsic EL system where an electric field excited luminescence is conducted accompanied by movement of carrier in the luminescent layer and a carrier injection EL system where an electric field excited luminescence is carried out by injecting carriers into a luminescent layer.
In addition, EL-devices may be classified into other two classes, depending on the structure of the luminescent layer, i.e. a thin film type having a thin film composed of a material of EL function as a luminescent layer and a powder type having a luminescent layer composed of a material of EL function dispersed in a binder.
As the material of EL function, there have been heretofore inorganic metal materials such as ZnS containing Mn, Cu, ReF.sub.3 and (Re: rare earths) or the like as an activating agent, and the like.
In the case of a thin film type EL device, the structure is suitable for the following purposes, that is, a thin luminescent layer can be formed so as to sufficiently shorten the distance between the electrodes and a strong electric field can be formed in the luminescent layer so as to produce a good luminescence of high luminance even by a low voltage driving. However, where the above-mentioned inorganic metal material such as ZnS is used to form a thin film type luminescent layer by a thin film forming method such as vapor deposition and the like and a thin film type EL device is fabricated, the manufacturing cost is very high and, in addition, it is very difficult to form a luminescent layer composed of a unform thin film of a large area and therefore, it is not possible to produce EL-devices of good quality and large area by mass production.
On the contrary, as an EL-device which is suitable for mass production and is inexpensive, there are known organic powder type EL-devices of an intrinsic EL system where the above-mentioned EL intrinsic material, mainly compound of ZnS, is dispersed in an organic binder to form a luminescent layer.
However, in the powder type EL-device, when the luminescent layer is made thin, defects such as pinhole and the like are liable to be formed in the luminescent layer. Thus, in view of the limitation due to the structure, it is difficult to make the luminescent layer thinner than a certain thickness for enhancing sufficiently the luminescent characteristics, and therefore sufficient luminescence, in particular, a high luminance, can not be obtained. Further, since the thickness of the luminescent layer becomes relatively thick, power consumption is disadvantageously large for generating a strong electric field.
For the purpose of generating a stronger electric field in the luminescent layer of the powder type EL-device, Japanese Patent Application Laid-open No. 172891/1983 discloses an improved EL-device comprising an intermediate dielectric layer composed of a polymer of vinylidene fluoride in a luminescent layer of powder type.
However, satisfactory luminance and desirably low power consumption have not been achieved.
On the contrary, in place of conventional metallic or inorganic materials, it has been recently contemplated to employ organic compound materials which can be formed into a thin film of high precision by utilizing various thin film forming methods, and control their chemical structures and high order structures to use them as optical and electronics materials in a form of electrochromic device, piezoelectric device, pyroelectric device, nonlinear optical device, ferroelectric liquid crystal or the like. In addition, it is also expected to use such organic materials as a material for constituting a luminescent layer for EL-devices.
Among them, an organic materials for a luminescent layer of EL-devies, there are known anthracene, pyrene, perylene, their derivatives and the like. Japanese Patent Application Laid-open No. 35587/1977 discloses an EL-device of carrier injection type where a monomolecular layer built-up film of the above-mentioned materials is used as a luminescent layer.
However, in this EL-device though the luminescent layer is formed by a thin film of high precision, the density of carriers, that is, electrons and holes, is so small that the excitation probability of the functional molecules due to movement and recombination of carriers is low and thereby, an efficient luminescence can not be produced, and, in particular, the power consumption and luminance are not yet satisfactory.